<template>
    <ul class="list">
        <li
         class="item" 
         v-for="item of letters" 
         :key="item" 
         :ref="item"
         @touchstart.prevent="handleTouchStart" 
         @touchmove="handleTouchMove"
         @touchend="handleTouchStart"
         @click="handleLetterClick"
         >
         {{item}}
         </li>
    </ul>
</template>

<script>
export default {
  name: 'CityAlphabet',
  props: {
      cities:Object
  },
  computed: {
      letters () {
          const letters = []
          for (let i in this.cities) {
              letters.push(i)
          }
          return letters
      }
  },
  data () {
      return {
          touchStatus: false,
          startY:0,
          timer: null
      }
  },
  updated () {
    this.startY = this.$refs['A'][0].offsetTop
  },
  methods: {
      handleLetterClick (e) {
          this.$emit('change',e.target.innerText)
      },
      handleTouchStart () {
          this.touchStatus = true
      },
      handleTouchMove (e) {
        if (this.touchStatus) {
            if (this.timer) {
            clearTimeout(this.timer)
            }
        this.timer = setTimeout(() => {
          const touchY = e.touches[0].clientY - 79
          const index = Math.floor((touchY - this.startY) / 20)
          if (index >= 0 && index < this.letters.length) {
            this.$emit('change', this.letters[index])
          }
        }, 16)
      }
    },
      handleTouchEnd () {
          this.touchStatus = false
      }
  }
}
</script>

<style lang="stylus" scoped>
@import '~styles/varibles.styl'
   .list{
        display: flex
        flex-direction: column
        justify-content: center
        position: absolute
        top: 1.58rem
        right: 0
        bottom: 0
        width: .4rem
   }
   .item{
        line-height: .4rem
        text-align: center
        color: $bgColor
   }
</style>